Power Cubes are used as a stash of XM that you can use to recharge your XM bar. You can obtain Power Cubes by hacking portals and redeeming passcodes. You can only use Power Cubes that are equal or lower than your Level. They are very useful because now almost every action in Ingress uses more and more XM.

Photos:

L1 Power Cube

L2 Power Cube

L3 Power Cube

L4 Power Cube

L5 Power Cube

XM gain:

Every power cube has its level * 1000 XM in them, and here is complete list: